Clang: Make diagnostic tooltips consistent

Fix that triggering a diagnostic tooltip from the diagnostic
location/range itself (underlined text) did not show the icon on the
left and the actions/toolbuttons on the right in the tooltip.

Instead of showing the tooltip content itself, request the tooltip for
the corresponding text mark to get the extra decoration and actions.

Change-Id: I5e94aca117a761f7a798d4f4b33db6e386e54d84
Reviewed-by: David Schulz <david.schulz@qt.io>
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>

src/plugins/clangcodemodel/clangtextmark.h

@@ -44,7 +44,9 @@ public:
                   const RemovedFromEditorHandler &removedHandler,
                   bool fullVisualization);
 
+    ClangBackEnd::DiagnosticContainer diagnostic() const { return m_diagnostic; }
     void updateIcon(bool valid = true);
+
 private:
     bool addToolTipContent(QLayout *target) const override;
     void removedFromEditor() override;